1,682 research outputs found
First Principles Study of CaFe2As2 "Collapse" Under Pressure
We perform first principles calculations on CaFe2As2 under hydrostatic
pressure. Our total energy calculations show that though the striped
antiferromagnetic (AFM) orthorhombic (OR) phase is favored at P=0, a
non-magnetic collapsed tetragonal (cT) phase with diminished c-parameter is
favored for P > 0.36 GPa, in agreement with experiments. Rather than a
mechanical instability, this is an enthalpically driven transition from the
higher volume OR phase to the lower volume cT phase. Calculations of electronic
density of states reveal pseudogaps in both OR and cT phases, though As(p)
hybridization with Fe(d) is more pronounced in the OR phase. We provide an
estimate for the inter-planar magnetic coupling. Phonon entropy considerations
provide an interpretation of the finite temperature phase boundaries of the cT
phase.Comment: 4 pages, 4 figures, 1 Tabl
Lower order terms in Szego type limit theorems on Zoll manifolds
This is a detailed version of the paper math.FA/0212273. The main motivation
for this work was to find an explicit formula for a "Szego-regularized"
determinant of a zeroth order pseudodifferential operator (PsDO) on a Zoll
manifold. The idea of the Szego-regularization was suggested by V. Guillemin
and K. Okikiolu. They have computed the second term in a Szego type expansion
on a Zoll manifold of an arbitrary dimension. In the present work we compute
the third asymptotic term in any dimension. In the case of dimension 2, our
formula gives the above mentioned expression for the Szego-redularized
determinant of a zeroth order PsDO. The proof uses a new combinatorial
identity, which generalizes a formula due to G.A.Hunt and F.J.Dyson. This
identity is related to the distribution of the maximum of a random walk with
i.i.d. steps on the real line. The proof of this combinatorial identity
together with historical remarks and a discussion of probabilistic and
algebraic connections has been published separately.Comment: 39 pages, full version, submitte
The two-angle model and the phase diagram for Chromatin
We have studied the phase diagram for chromatin within the framework of the
two-angle model. Rather than improving existing models with finer details our
main focus of the work is getting mathematically rigorous results on the
structure, especially on the excluded volume effects and the effects on the
energy due to the long-range forces and their screening. Thus we present a
phase diagram for the allowed conformations and the Coulomb energies
Liquid-liquid transition in supercooled silicon determined by first-principles simulation
First principles molecular dynamics simulations reveal a liquid-liquid phase
transition in supercooled elemental silicon. Two phases coexist below
. The low density phase is nearly tetra-coordinated, with a
pseudogap at the Fermi surface, while the high density phase is more highly
coordinated and metallic in nature. The transition is observed through the
formation of van der Waals loops in pressure-volume isotherms below .Comment: 9 pages 4 figure
Super-Radiance and the Unstable Photon Oscillator
If the damping of a simple harmonic oscillator from a thermally random force
is sufficiently strong, then the oscillator may become unstable. For a photon
oscillator (radiatively damped by electric dipole moments), the instability
leads to a low temperature Hepp-Lieb-Preparata super-radiant phase transition.
The stable oscillator regime is described by the free energy of the
conventional Casimir effect. The unstable (strongly damped) oscillator has a
free energy corresponding to Dicke super-radiance.Comment: 6 pages ReVTeX 2 figures *.ep
Entanglement and particle correlations of Fermi gases in harmonic traps
We investigate quantum correlations in the ground state of noninteracting
Fermi gases of N particles trapped by an external space-dependent harmonic
potential, in any dimension. For this purpose, we compute one-particle
correlations, particle fluctuations and bipartite entanglement entropies of
extended space regions, and study their large-N scaling behaviors. The
half-space von Neumann entanglement entropy is computed for any dimension,
obtaining S_HS = c_l N^(d-1)/d ln N, analogously to homogenous systems, with
c_l=1/6, 1/(6\sqrt{2}), 1/(6\sqrt{6}) in one, two and three dimensions
respectively. We show that the asymptotic large-N relation S_A\approx \pi^2
V_A/3, between the von Neumann entanglement entropy S_A and particle variance
V_A of an extended space region A, holds for any subsystem A and in any
dimension, analogously to homogeneous noninteracting Fermi gases.Comment: 15 pages, 22 fig
Real and virtual strange processes
Following notions of quantum mechanics as interpreted by the Copenhagen School, we make a distinction between measurements involving one or two virtual K mesons. New predictions result for the period of K oscillations at the Phi Factory
- …